1. Field of the Invention
The present invention relates to a magnetic recording-reproducing apparatus.
2. Description of the Related Art
With improvement in the processing speed of an electronic computer achieved in recent years, further improvements in the operating speed and in the information storing density continue to be required for a magnetic memory apparatus (HDD) performing the information storing function and the information reproducing function. However, it is said that there is a physical limit in the further improvement in the information storing density, and whether the requirement noted above continues to be satisfied is said to be questionable.
The magnetic recording medium included in the HDD comprises as a recording layer a magnetic layer formed of an aggregate of fine magnetic particles. In order to achieve a high-density recording in such a magnetic recording medium, it is desirable for the magnetic domains recorded in the magnetic layer to be small. In order to allow the small recording domains to be distinguishable, it is necessary for the boundaries of the domains to be smooth. For this purpose, it is necessary to make magnetic particles as small as possible. It should also be noted that, linkage of magnetization transition between the adjacent magnetic particles causes disturbance in the boundaries between the adjacent domains. Therefore, it is necessary to arrange a nonmagnetic material between the adjacent magnetic particles so as to magnetically separate the magnetic particles and, thus, to prevent exchange coupling interaction from being exerted between the adjacent magnetic particles.
Also, in order to read magnetic information recorded in the magnetic recording medium, it is necessary to enhance interaction between the magnetic head of the HDD and the magnetic layer of the magnetic recording medium. For enhancing the interaction noted above, it is also necessary to decrease the thickness of the magnetic layer included in the magnetic recording medium.
Under the circumstances, it is necessary to decrease the volume of the magnetization reversal unit, which is substantially equal to the volume of the magnetic particle, of the magnetic material constituting the magnetic layer of the magnetic recording medium.
It should be noted, however, that, if the magnetization reversal unit is miniaturized, the magnetic anisotropy energy of the unit, which is equal to the product of the magnetic anisotropy energy density Ku and the volume V of the magnetization reversal unit, is rendered smaller than the thermal fluctuation energy, resulting in failure to maintain the domains. This is called a thermal fluctuation phenomenon. Also, the physical limit of the recording density that is caused mainly by the thermal fluctuation phenomenon is called a thermal fluctuation limit.
In order to prevent the magnetization reversal caused by the thermal fluctuation, it is conceivable to make the magnetic anisotropy energy of the magnetic layer higher than the thermal fluctuation energy. However, if the magnetic anisotropy energy of the magnetic layer is increased, the coercivity of the magnetic layer, which is substantially proportional to the magnetic anisotropy energy, is also increased so as to give rise to the problem that recording cannot be performed under the magnetic field that can be produced by a writing head available nowadays.
As described above, the prior art is defective in that, even if the magnetization reversal unit is miniaturized for performing a high-density recording, the recording density is limited by the thermal fluctuation limit. On the other hand, if a magnetic material having high magnetic anisotropy energy is used for forming the recording layer in order to prevent the magnetization reversal caused by the thermal fluctuation, the coercivity is rendered excessively high so as to give rise to the problem that the recording cannot be performed under the magnetic field that can be produced by the writing head available nowadays.